Unix was the first production operating system to be ported between differing processor families (Version 6 Unix, 1976-77). Today, Unix is routinely ported to every new machine powerful enough to sport a memory-management unit. Unix applications are routinely moved between Unixes running on wildly differing hardware; in fact, it is unheard of for a port to fail.

Portability has always been one of Unix's principal advantages. Unix programmers tend to write on the assumption that hardware is evanescent and only the Unix API is stable, making as few assumptions as possible about machine specifics such as word length, endianness or memory architecture. In fact, code that is hardware-dependent in any way that goes beyond the abstract machine model of C is considered bad form in Unix circles, and only really tolerated in very special cases like operating system kernels.

Unix programmers have learned that it is easy to be wrong when anticipating that a software project will have a short lifetime.^[141] Thus, they tend to avoid making software dependent on specific and perishable technologies, and to lean heavily on open standards. These habits of writing for portability are so ingrained in the Unix tradition that they are applied even to small single-use projects that are thought of as throwaway code. They have had secondary effects all through the design of the Unix development toolkit, and on programming languages like Perl and Python and Tcl that were developed under Unix.

The direct benefit of portability is that it is normal for Unix software to outlive its original hardware platform, so tools and applications don't have to be reinvented every few years. Today, applications originally written for Version 7 Unix (1979) are routinely used not merely on Unixes genetically descended from V7, but on variants like Linux in which the operating system API was written from a Unix specification and shares no code with the Bell Labs source tree.

The indirect benefits are less obvious but may be more important. The discipline of portability tends to exert a simplifying influence on architectures, interfaces, and implementations. This both increases the odds of project success and reduces life-cycle maintenance costs.

In this chapter, we'll survey the scope and history of Unix standards. We'll discuss which ones are still relevant today and describe the areas of greater and lesser variance in the Unix API. We'll examine the tools and practices that Unix developers use to keep code portable, and develop some guides to good practice.